PGD-Always-ON

PGD-Always-ON#

Note

This architecture is for Postgres Distributed 5 only. If you require PGD 4 or 3.7 please use BDR-Always-ON .

EDB Postgres Distributed 5 in an Always-ON configuration, suitable for use in test and production.

This architecture requires an EDB subscription. All software will be sourced from Configuring EDB Repos 2.0 repositories .

Cluster configuration#

Overview of configuration options#

An example invocation of tpaexec configure for this architecture is shown below.

tpaexec configure ~/clusters/pgd-ao \
         --architecture PGD-Always-ON \
         --edb-postgres-extended 15 \
         --platform aws --instance-type t3.micro \
         --distribution Debian \
         --pgd-proxy-routing global \
         --location-names dc1 dc2 dc3 \
         --witness-only-location dc3 \
         --data-nodes-per-location 2

You can list all available options using the help command.

tpaexec configure --architecture PGD-Always-ON --help

The table below describes the mandatory options for PGD-Always-ON and additional important options. More detail on the options is provided in the following section.

Mandatory Options#

Options

Description

--architecture (-a)

Must be set to PGD-Always-ON

Postgres flavour and version (e.g. --postgresql 15)

A valid Postgres flavour and version .

--pgd-proxy-routing

Must be either global or local.

Additional Options#

Options

Description

Behaviour if omitted

--platform

One of aws, docker, bare.

Defaults to aws.

--location-names

A space-separated list of location names. The number of locations is equal to the number of names supplied.

TPA will configure a single location with three data nodes.

--witness-only-location

A location name, must be a member of location-names.

No witness-only location is added.

--data-nodes-per-location

The number of data nodes in each location, must be at least 2.

Defaults to 3.

--add-proxy-nodes-per-location

The number of proxy nodes in each location.

PGD-proxy will be installed on each data node.

--enable-camo

Sets two data nodes in each location as CAMO partners.

CAMO will not be enabled.

--bdr-database

The name of the database to be used for replication.

Defaults to bdrdb.

--enable-pgd-probes

Enable http(s) api endpoints for pgd-proxy such as health/is-ready to allow probing proxy's health.

Disabled by default.

--proxy-listen-port

The port on which proxy nodes will route traffic to the write leader.

Defaults to 6432

--proxy-read-only-port

The port on which proxy nodes will route read-only traffic to shadow nodes.

Defaults to 6433

More detail about PGD-Always-ON configuration#

A PGD-Always-ON cluster comprises a number of locations, preferably odd, each with the same number of data nodes, again preferably odd. If you do not specify any --location-names , the default is to use a single location with three data nodes.

Location names for the cluster are specified as --location-names dc1 dc2 … . A location represents an independent data centre that provides a level of redundancy, in whatever way this definition makes sense to your use case. For example, AWS regions, your own data centres, or any other designation to identify where your servers are hosted.

Use --data-nodes-per-location N to specify the number of data nodes in each location. The minimum number is 2, the default is 3.

If you specify an even number of data nodes per location, TPA will add an extra witness node to each location automatically. This retains the ability to establish reliable consensus while allowing cost savings (a witness has minimal hardware requirements compared to the data nodes).

A cluster with only two locations would entirely lose the ability to establish global consensus if one of the locations were to fail. We recommend adding a third witness-only location (which contains no data nodes, only a witness node, again used to reliably establish consensus). Use --witness-only-location loc to designate one of your locations as a witness.

By default, every data node (in every location) will also run PGD-Proxy for connection routing. To create separate PGD-Proxy instances instead, use --add-proxy-nodes-per-location 3 (or however many proxies you want to add).

Depending on your use-case, you must specify --pgd-proxy-routing local or global to configure how PGD-Proxy will route connections to a write leader. Local routing will make every PGD-Proxy route to a write leader within its own location (suitable for geo-sharding applications). Global routing will make every proxy route to a single write leader, elected amongst all available data nodes across all locations.

You may optionally specify --bdr-database dbname to set the name of the database with BDR enabled (default: bdrdb).

You may optionally specify --enable-camo to set two data nodes in each region as CAMO partners.

You may optionally specify --enable-pgd-probes [{http, https}] to enable http(s) api endpoints that will allow to easily probe proxy’s health.

You may also specify any of the options described by Cluster configuration .

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